1. Field of the Invention
The present invention relates to a technique for subjecting semiconductors to each kind of annealing by irradiating the semiconductors with laser light.
2. Description of the Prior Art
Heretofore, techniques are known for subjecting semiconductors to each kind of annealing by irradiating the semiconductors with laser light. For example, the following techniques are known; a technique for transforming an amorphous silicon film (a-Si film) formed on a glass substrate by the plasma CVD into a crystalline silicon film by irradiating the amorphous silicon film with laser light; and an annealing technique after impurity ion doping, or the like. As each kind of annealing technique using such laser light and an apparatus for laser light irradiation, there is a technique described in Japanese Unexamined Patent Application No. Hei 6-51238 filed by the applicant of the present invention.
Since each kind of annealing treatment using laser light does not cause thermal damage to a base substrate, the treatment becomes a useful technique in the case where a material that is weak to heat such as a glass substrate or the like is used as the substrate. However, there is a problem in that it is difficult to keep the annealing effect on a constant level at all times. Further, when an amorphous silicon film is crystallized by irradiating the amorphous silicon film with laser light, it is difficult to constantly obtain a favorable crystallinity that is required. Thus, a demand has been made on a technique for stably obtaining a crystalline silicon film having a more favorable crystallinity.
An object of the present invention is to solve at least one or more of the problems described in the following items:
(1) to enable providing a constant effect at all times in techniques of annealing semiconductors by irradiating the semiconductors with laser light; and
(2) to further heighten the crystallinity of a crystalline silicon film obtained by irradiating an amorphous silicon film with laser light.
A first embodiment of the invention disclosed herein is a method comprising the steps of: heat-treating an amorphous silicon film to crystallize it; and irradiating the crystallized silicon film with laser light. This method is characterized in that during the irradiation of the laser light, the sample is maintained within xc2x1100xc2x0 C. of the temperature of the heat-treatment.
In the first embodiment constructed as described above, the temperature of the heat treatment performed during the crystallization step can be selected to be 450-750xc2x0 C. The upper limit of this temperature is restricted by the highest tolerable temperature of the substrate. Where a substrate made of glass is used, the upper limit is about 600xc2x0 C. Where the productivity is taken into account, this temperature is preferably above 550xc2x0 C. Therefore, where a glass substrate is employed, it is desired to perform a heat treatment at a temperature of about 550-600xc2x0 C.
During the laser irradiation, the heating temperature is preferably set to about 550-600xc2x0 C. Heating starting from a temperature of about 450xc2x0 C. can be put into practical use. Accordingly, the heating temperature preferably lies in the range of 550xc2x0 C.xc2x1100xc2x0 C.
A second embodiment of the invention disclosed herein is a method comprising the steps of: heat-treating an amorphous silicon film at a temperature lower than 600xc2x0 C. to crystallize the amorphous silicon film; and irradiating the crystallized silicon film with laser light. This method is characterized in that during the laser irradiation, the sample is maintained within xc2x1100xc2x0 C. of the temperature of the heat treatment.
A third embodiment of the invention disclosed herein is a method comprising the steps of: heat-treating an amorphous silicon film to crystallize it; implanting impurity ions into at least a region of the crystallized silicon film; and irradiating the ion-implanted region with laser light. This method is characterized in that during the laser irradiation, the sample is maintained within xc2x1100xc2x0 C. of the temperature of the heat treatment.
A fourth embodiment of the invention disclosed herein is a method comprising the steps of: heat-treating an amorphous silicon film to crystallize it; implanting impurity ions into at least a region of the crystallized silicon film; and irradiating the ion-implanted region with laser light. This method is characterized in that during the laser irradiation, the sample is maintained within xc2x1100xc2x0 C. of the temperature of the heat treatment.
A fifth embodiment of the invention disclosed herein is a method comprising the steps of: irradiating an amorphous silicon film with a laser beam having a linear cross section while moving the laser beam in steps from one side of the amorphous silicon film to opposite side to crystallize irradiated regions in succession. This method is characterized in that the laser irradiation is performed while heating the irradiated surface above 450xc2x0 C.
In the fifth embodiment constructed as described above, the laser beam of the linear cross section is moved in steps and made to impinge on the film. Consequently, the required regions can be effectively irradiated with the laser light. Normally, the temperatures of irradiated surfaces are limited to about 600xc2x0 C. However, these temperature are restricted by the material of the substrate. Higher temperatures may also be used.
A sixth embodiment of the invention disclosed herein is a method comprising the steps of: introducing a metal element for promoting crystallization into an amorphous silicon film; heat-treating the amorphous silicon film to crystallize it; and irradiating the crystallized silicon film with laser light. This method is characterized in that during the laser irradiation, the sample is maintained within xc2x1100xc2x0 C. of the temperature of the heat treatment.
In the sixth embodiment constructed as described above and in the following seventh through tenth embodiments, the metal element for promoting crystallization is one or more elements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Zn, Ag, and Au. Among these metal elements, nickel is the element producing the most conspicuous effect.
In the above-described configurations, the heat treatment temperature can be selected to lie within the range of from 450xc2x0 C. to 750xc2x0 C. The upper limit of this temperature is restricted by the highest tolerable temperature of the substrate. Where a glass substrate is used, the upper limit is roughly 600xc2x0 C. Where the productivity is taken into consideration, this temperature is preferably higher than 550xc2x0 C. Accordingly, where a glass substrate is employed, the heat treatment is preferably performed at a temperature of about 550xc2x0 C.-600xc2x0 C.
Furthermore, during the laser irradiation, the heat treatment temperature is preferably about 550-600xc2x0 C. Heating starting from a temperature of about 450xc2x0 C. can be put into practical use. In consequence, it is desired to heat the substrate within the temperature range of from 550xc2x0 C.xc2x1100xc2x0 C.
A seventh embodiment of the invention disclosed herein is a method comprising the steps of: introducing a metal element for promoting crystallization into an amorphous silicon film; heat-treating the amorphous silicon film at a temperature lower than 600xc2x0 C. to crystallize it; and irradiating the crystallized silicon film with laser light. This method is characterized in that during the laser irradiation, the sample is maintained within xc2x1100xc2x0 C. of the temperature of the heat treatment.
An eighth embodiment of the invention disclosed herein is a method comprising the steps of: introducing a metal element for promoting crystallization into an amorphous silicon film; heat-treating the amorphous silicon film to crystallize it; implanting impurity ions into at least a region of the crystallized silicon film; and irradiating the ion-implanted region with laser light. This method is characterized in that during the laser irradiation, the sample is maintained within xc2x1100xc2x0 C. of the temperature of the heat treatment.
A ninth embodiment of the invention disclosed herein is a method comprising the steps of: introducing a metal element for promoting crystallization into an amorphous silicon film; heat-treating the amorphous silicon film to crystallize it; implanting impurity ions into at least a region of the crystallized silicon film; and irradiating the ion-implanted region with laser light. This method is characterized in that during the laser irradiation, the sample is maintained within xc2x1100xc2x0 C. of the temperature of the heat treatment.
A tenth embodiment of the invention disclosed herein is a method comprising the steps of: introducing a metal element for promoting crystallization into an amorphous silicon film; irradiating the amorphous silicon film with a laser beam having a linear cross section while moving the laser beam in steps from one side of the amorphous silicon film to opposite side to crystallize irradiated regions in succession. This method is characterized in that the laser irradiation is performed while heating the irradiated surface above 450xc2x0 C.
In the tenth embodiment constructed as described above, the laser beam of the linear cross section is moved in steps and made to impinge on desired regions. Consequently, the desired regions can be effectively irradiated with the laser light. Normally, the temperatures of irradiated surfaces are limited to about 600xc2x0 C. However, these temperature are restricted by the material of the substrate. Higher temperatures may also be used.
A laser processing method according to an eleventh embodiment of the invention consists of irradiating a silicon film formed on a glass substrate with laser light. This method is characterized in that during the laser irradiation, the silicon film is heated at a temperature which is higher than 455xc2x0 C. and lower than strain point of the glass substrate.
A laser processing method according to a twelfth embodiment of the invention comprises the steps of: irradiating a silicon film formed on a glass substrate with laser light; and then heating the silicon film at a temperature which is higher than 500xc2x0 C. and lower than strain point of the glass substrate. This method is characterized in that during the laser irradiation, the silicon film is heated at a temperature which is higher than 455xc2x0 C. and lower than the strain point of the glass substrate.
A laser processing method according to a thirteenth embodiment of the invention consists of irradiating a silicon film formed on a glass substrate with laser light. This method is characterized in that during the laser irradiation, the silicon film is heated at a temperature of 550xc2x0 C.xc2x130xc2x0 C.
A laser processing method according to a fourteenth embodiment of the invention comprises the steps of: irradiating a silicon film formed on a glass substrate with laser light; and then heating the silicon film at a temperature of 550xc2x0 C.xc2x130xc2x0 C. This method is characterized in that during the laser irradiation, the silicon film is heated at a temperature of 550xc2x0 C.xc2x130xc2x0 C.
A laser processing method according to a fifteenth embodiment of the invention comprises the steps of: forming a silicon film on a glass substrate; heating the silicon film up to a desired temperature; and irradiating the silicon film with laser light while maintaining the desired temperature. This method is characterized in that the desired temperature is higher than 500xc2x0 C. and lower than strain point of the glass substrate.
A laser processing method according to a sixteenth embodiment of the invention comprises the steps of: forming a silicon film on a glass substrate; making a first heat treatment of said amorphous silicon film to crystallize it; irradiating the crystallized silicon film with laser light; and then making a second heat treatment of the silicon film. This method is characterized in that one or both of the first and second heat treatments are made at a temperature which is higher than 500xc2x0 C. and lower than strain point of the glass substrate. This method is also characterized in that the laser irradiation step is performed while heating the substrate at a temperature which is higher than 455xc2x0 C. and lower than the strain point of the glass substrate.
A laser processing method according to a seventeenth embodiment of the invention comprises the steps of: forming an amorphous silicon film on a glass substrate; introducing a metal element for promoting crystallization of silicon into the amorphous silicon film; making a first heat treatment of the amorphous film; then irradiating the crystallized silicon film with laser light; and then making a second heat treatment of the silicon film. This method is characterized in that one or both of the first and second heat treatments are made at a temperature which is higher than 500xc2x0 C. and lower than strain point of the glass substrate. This method is also characterized in that the laser irradiation step is performed while heating the substrate at a temperature which is higher than 455xc2x0 C. and lower than the strain point of the glass substrate.
In the laser processing methods according to the eleventh through seventeenth embodiments described above, a silicon film formed on a glass substrate is irradiated with laser light. During the laser irradiation, the substrate is heated at a temperature which is higher than 455xc2x0 C. and lower than strain point of the glass substrate.
The silicon film formed on the glass substrate can be an amorphous or crystalline silicon film directly formed on the glass substrate. Alternatively, an insulating film such as a silicon oxide film or silicon nitride film is formed as a buffer film on the glass substrate. An amorphous or crystalline silicon film is formed on the buffer film.
The substrate is heated above 455xc2x0 C. during the laser irradiation to enhance the annealing effect of the laser irradiation. The silicon film is irradiated with laser light to impart energy to the silicon film. This energy crystallizes the silicon film, improves the crystallinity of the silicon film, or activates impurities contained in the silicon film. The heating is used together with the laser irradiation. This can enhance the effect of the laser irradiation.
We irradiated an amorphous silicon film with KrF excimer laser light having a wavelength of 248 nm to crystallize the amorphous silicon film. This amorphous silicon film was formed on a buffer film of silicon oxide film, which was, in turn, formed on a glass substrate. FIG. 22 shows the relation of the Raman intensity (relative value) of the silicon film to the energy density of incident laser light. The Raman intensity (relative value) is the ratio of the Raman intensity of the silicon film to the Raman intensity of single-crystal wafer. It follows that as the Raman intensity (relative value) is increased, the crystallinity is improved. It can be seen from the graph of FIG. 22 that a silicon film of higher crystallinity is obtained by heating the substrate (sample) while irradiating it with laser light, if the intensity of the laser light remains the same.
FIG. 23 shows the relation of the half-value widths (relative values) of Raman spectra to the energy densities of incident light. The half-value width of a Raman spectrum is the ratio of the width giving a half value of the peak of the Raman spectrum to the width of the Raman spectrum obtained from the single-crystal wafer. It follows that as this half-value width is reduced, the obtained silicon film has higher crystallinity.
As can be seen from the graph of FIG. 23, a silicon film having excellent crystallinity is obtained by heating the film while irradiating it with laser light at the same time. Our experiments have shown that the temperature of the heating conducted simultaneously with the laser irradiation is set higher than 455xc2x0 C., preferably above 500xc2x0 C. More preferably, the temperature is higher than 550xc2x0 C. Where the heating is done above 500xc2x0 C., conspicuous effects are obtained.
One method of heating the substrate can use a heater mounted in a holder or stage holding the substrate. Another method consists of heating the irradiated surface by infrared light or the like. Correctly, the heating temperature is the measured temperature of the irradiated surface. However, if slight error is tolerated, then the measured temperature of the substrate can be used as the heating temperature.
The heating done simultaneously with the laser irradiation is preferably carried out below the strain point of the glass substrate, because the substrate is prevented from warping or shrinking in spite of the heating. For example, Corning 7059 glass which is often used as the substrate of an active matrix liquid crystal display has a strain point of 593xc2x0 C. In this case, it is desired to conduct the heat treatment at a temperature lower than 593xc2x0 C.
Furthermore, it has been empirically known that during the laser irradiation, if the substrate is heated at a temperature of 550xc2x0 C.xc2x130xc2x0 C., then desirable results arise.
Especially, if the silicon film is crystallized by heating before the laser irradiation, then conspicuous results are obtained. In the laser processing method according to the sixteenth embodiment described above, the amorphous silicon film formed on the glass substrate is first crystallized by heating. Then, the crystallinity is further enhanced by laser irradiation. Subsequently, the film is heat-treated. In this way, the defect density in the obtained silicon film is reduced.
In the laser processing method according to the seventeenth embodiment, a catalytic element for promoting crystallization of the amorphous silicon film is introduced into the silicon film. Then, a heat treatment is made to crystallize the amorphous film. The metal element for promoting crystallization can be one or more elements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au. Especially, where nickel (Ni) is used, a crystalline silicon film can be obtained by performing a heat treatment at a temperature of 550xc2x0 C.xc2x130xc2x0 C. for about 4 hours.
One method of introducing the above-described element consists of forming either a layer of the metal element or a layer containing the metal element in contact with the surface of the amorphous silicon film by sputtering, evaporation, or CVD techniques. Another method of introducing the above-described element consists of applying a liquid solution containing the metal element to the surface of the amorphous silicon film and holding the metal element in contact with the surface of the amorphous silicon film.
The amount of the metal element introduced is so set that the concentration of the metal element in the silicon film is 1xc3x971016 cmxe2x88x923 to 5xc3x971019 cmxe2x88x923, for the following reason. If the concentration of the metal element is less than 1xc3x971016 cmxe2x88x923, then the desired effect cannot be obtained. Conversely, if the concentration of the metal element is in excess of 5xc3x971019 cmxe2x88x923, then the electrical characteristics of the semiconductor, or the obtained crystalline silicon film, are impaired. That is, the electrical characteristics of the film acting as a metal become more conspicuous.
Nickel element was introduced into several samples of amorphous silicon film. The samples were heat-treated to crystallize them. In this way, crystalline silicon films were derived. The spin densities in the films were measured. The results are listed in FIG. 24. It can be understood that the spin density in each film is a measure of the defect density in the film.
In FIG. 24, samples 1, 2, and 5 underwent only heat treatment after introduction of nickel element. Sample 3 underwent laser irradiation (LI) after heat treatment. Sample 4 underwent laser irradiation (LI) after heat treatment. Then, sample 4 was subjected to heat treatment. As can be seen from FIG. 24, sample 4 has the lowest spin density, it being noted that sample 4 underwent heat treatment after laser irradiation (LI).
In this way, heat treatment conducted after laser irradiation is quite effective in reducing the defect density in the film. If the temperature of the heat treatment performed after the laser irradiation is set above 500xc2x0 C., then desirable results are produced. The upper limit of the temperature is restricted by the strain point of the glass substrate.
A laser processing system according to an eighteenth embodiment of the invention comprises: a conveyance chamber having a means for transporting a substrate; a first heating chamber having a means for heating the substrate; a second heating chamber for heating the substrate; and a laser processing chamber having a means for directing laser light to the substrate. The first heating chamber, the second heating chamber, and the laser processing chamber are connected together via the conveyance chamber. In the first heating chamber, the substrate is heated at a desired temperature. In the laser processing chamber, the substrate which was heated in the first heating chamber is irradiated with the laser light while heated. In the second heating chamber, the substrate which was irradiated with the laser light in the laser processing chamber is heat-treated.
Examples of system having the above-described structure are shown in FIGS. 18-20. In FIG. 18, indicated by reference numeral 301 is a conveyance chamber having a means 314 (robot arm) for transporting a substrate 315. Heating chambers 305 and 302 have means for heating the substrate. A laser processing chamber 304 has means for directing laser light to the substrate.
A laser processing system according to a nineteenth embodiment of the invention comprises a means for irradiating a substrate with laser light and a means for rotating the substrate through 90 degrees. This system is characterized in that the laser light has a linear cross section.
A laser processing system according to a twenty-first embodiment of the invention comprises a means for irradiating a substrate with laser light and a means for rotating the substrate through 90 degrees. This system is characterized in that the laser light has a linear cross section, and that this laser light of the linear cross section is scanned at right angles to longitudinal direction of the cross section of the laser light and directed to the substrate. The substrate is rotated through 90 degrees by the rotating means. Thus, the laser light of the linear cross section is scanned from an orientation differing by 90 degrees from the previous orientation and directed to the substrate.
A laser processing system according to a twenty-second embodiment of the invention comprises a means for irradiating a substrate with laser light and a means for rotating the substrate. This system is characterized in that the laser light has a linear cross section.
A laser processing system according to a twenty-third embodiment of the invention comprises a means for irradiating a substrate with laser light and a means for rotating the substrate. This system is characterized in that the laser light has a linear cross section, and that this laser light is scanned at right angles to longitudinal direction of the cross section of the laser light and directed to the substrate. The substrate is rotated by the rotating means so that the linear laser light is scanned at an orientation different from the previous orientation and directed to the substrate.
A laser processing system according to a twenty-fourth embodiment of the invention comprises: a laser light-irradiating chamber having means for producing laser light; a substrate-rotating chamber having a means for rotating a substrate; and a conveyance chamber connected to these two chambers and having a conveyance means for transporting the substrate. This system is characterized in that the laser light has a linear cross section, and that the linear laser light is scanned at right angles to longitudinal direction of cross section of the laser light and directed to the substrate. Once the substrate is irradiated with the laser light, the substrate is transported into the rotating chamber by the conveyance means and rotated by the rotating means. Then, the substrate is again transported into the laser light-irradiating chamber by the conveyance means. The substrate is again scanned with the laser light but at an angle different from the angle at which the laser light was emitted previously.
Examples of laser processing system having the above-described structure are shown in FIGS. 18-20. Systems shown in FIGS. 18-30 have means for producing laser light in a laser processing chamber 304. In FIG. 20, indicated by numeral 331 is a laser for emitting laser light. Also, there are provided means for rotating the substrate by 90 degrees in a chamber indicated by 303. Laser light from the laser 331 has a linear cross section whose longitudinal direction is directed from the front side of the sheet of FIG. 3 to the opposite side.
The substrate shown in FIG. 3 is placed on a stage 353. This stage is moved in the direction indicated by 354 so that the linear beam is scanned at right angles to the longitudinal direction of the beam. In the configuration shown in FIG. 3, the laser beam is scanned relative to the substrate by moving the substrate. Of course, the laser beam may be moved.
The laser irradiation can be repeated at least twice such that the direction of the scan of the linear laser beam is varied by 90 degrees from the direction of the previous scan. In this way, the whole desired surface can be uniformly irradiated with the laser light.
After the first laser irradiation step, the substrate is rotated through 90 degrees inside the chamber 303. Then, the second laser irradiation step is performed. This can enhance the uniformity of the effect of the laser irradiation. Of course, this scan can be repeated plural times.
Furthermore, the substrate can be rotated through 30 degrees. Three laser irradiation steps can be performed. Of course, the number of laser irradiation steps can be increased further. The angle through which the substrate is rotated can be set at will in view of the uniformity of the laser irradiation.
The twenty-fifth aspect of the present invention comprises:
a step of introducing into an amorphous silicon film a metal element which promotes the crystallization of the amorphous silicon film;
a step of heat treating the aforementioned amorphous silicon film to crystallize the amorphous silicon film; and
a step of irradiating with laser light the silicon film crystallized at the preceding steps;
wherein a sample is kept at a temperature within a range of xc2x1100xc2x0 C. from the temperature at the aforementioned heat treatment.
In the aforementioned structure (of all the aspects disclosed in the specification), as a metal element promoting the crystallization, one kind of metal element or a plurality of kinds thereof can be selected such metal elements such as Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Zn, Ag and Au. Of these metal elements, nickel is a metal element that can provide the most conspicuous effect.
In the aforementioned structure, as a temperature at the time of heat treatment, a temperature in the range of from 450 to 750xc2x0 C., C can be selected. An upper limit of this temperature is limited by the heat resistance temperature of the substrate. When a glass substrate is used as the substrate, about 600xc2x0 C. is considered to be the upper limit. Further, when the productivity is considered, it is desirable that this temperature is 550xc2x0 C. or more. Consequently, it follows that when the glass substrate is used, it is desirable to heat treat the glass substrate at about 550 to 600xc2x0 C.
It is desirable to set the heating temperature at the time of laser light irradiation to about 550 to 600xc2x0 C. However, heating at a temperature of about 450xc2x0 C. or higher is practical. Consequently, it is preferable to heat the glass substrate at a temperature within the scope of 550xc2x0 C.xc2x1100xc2x0 C.
Further, the twenty-sixth aspect according to the present invention comprises:
a step of introducing into an amorphous silicon film a metal element which promotes the crystallization of the amorphous silicon film;
a step of heat treating the aforementioned amorphous silicon film at 600xc2x0 C. or less to crystallize the amorphous silicon film; and
a step of irradiating with laser light the silicon film crystallized at the previous steps with a sample being kept at a temperature within a range of xc2x1100xc2x0 C. from the temperature at the time of the aforementioned heat treatment.
Further, the twenty-seventh aspect according to the present invention comprises:
a step of introducing into an amorphous silicon film a metal element which promotes the crystallization of the amorphous silicon film;
a step of heat treating the aforementioned amorphous silicon film to crystallize the amorphous silicon film;
a step of doping impurity ions into at least part of the silicon film crystallized at the preceding steps; and
a step of irradiating with laser light an area into which the aforementioned impurity ions are doped with a sample being kept at a temperature within a range of xc2x1100xc2x0 C. from the temperature at the time of the aforementioned heat treatment.
Further, another aspects according to the present invention comprises:
irradiating with laser light having a linear beam configuration an amorphous silicon film into which a metal element which promotes the crystallization of the amorphous silicon film is introduced by moving the amorphous silicon film successively from one side of the amorphous silicon film to other side; and
successively crystallizing an area irradiated with laser light;
wherein the aforementioned laser light irradiation is carried out by heating to 450xc2x0 C. or more a surface free from laser light irradiation.
In the aforementioned structure, a necessary area can be effectively irradiated with laser light by successively moving a linear beam to irradiate the area with the linear beam. Further, the condition of the temperature (heating temperature) on a surface to be irradiated with laser light is that the temperature is normally limited to about 600xc2x0 C. However, this temperature is limited by the material quality of the substrate. Otherwise, a much higher temperature may be set.